For many years, the hulking Riverview condo building has been a sought-after place to live in Cambridge, its 66 units of high-end housing offering residents an enviable perch overlooking the Charles, and, on the concrete decks that jut out toward the water, a prime spot for Head of the Charles Regatta watch parties. Many residents in the Mount Auburn Street structure are seniors — some who moved there in retirement, and others who have spent decades there. Most own, some rent.   Read the article…………………………….